Studies of Breeding Sandwich Terns Alistair J
Total Page：16
File Type：pdf, Size：1020Kb
Studies of breeding Sandwich Terns Alistair J. M. Smith Photographs by Jan van de Kam Plates 16-23 INTRODUCTION It is opportune that a series of Jan van de Kam's wonderful photo• graphs of Sandwich Terns Sterna sandvicensis at a Dutch breeding colony (plates 16-23) should appear at a time when the European population is threatened at both ends of its range. The effects of human disturbance and ground predators in Scottish colonies and of trapping for food in West Africa have been highlighted recently (Bourne and Smith 1974). The disastrous effects of pollution at Dutch colonies in the 1960's are well known (Rooth and Jonkers 1972) and, although these colonies have shown signs of recovery, only about 3,500 pairs bred in 1973 (B. Speek verbally). Apparently half of the west European population now breeds in Britain and Ireland, where there are possibly 12,000 pairs (Cramp et al. 1974). The Sandwich Tern is usually placed in the genus Sterna, but Moynihan (1959) related it to the crested 'Tkalasseus' terns and certainly its breeding behaviour seems more akin to that of such species as the Royal Tern S. maxima (Buckley and Buckley 1972) and Caspian Tern Hydroprogne caspia (Bergman 1953) than to that of the smaller Sterna terns. ARRIVAL AT BREEDING COLONIES The Sandwich Tern has a vast wintering range, extending from the north Mediterranean along the north and west coasts of Africa to the Cape and thence north-east as far as Mozambique (Miiller 1959, Langham 1971, C. J. Mead in litt.). Most breeding adults (three years and older) and some first- and second-year birds begin to return north from February. Adults assume breeding plumage from early February and I have seen aerial courtship from mid-February in Ghana. Once, in Sierra Leone, I encountered ground courtship as early as 20th January, while the birds were still in winter plumage. The first arrivals normally reach the south coast of England in March and it is not uncommon to find them at a breeding colony in northern Scotland at the end of March or early April. There is very little information on their behaviour at this time, but at Sands of Forvie, Grampian, their habit of visiting the colony immediately on arrival has given me an opportunity to study this 142 {Brit, Birds, 68: 142-156, April 1975} Sandwich Tern studies 143 stage. At first they tend to leave at dawn to fish along adjacent coasts, sometimes up to i£ km offshore. During the forenoon single birds, pairs or small groups only occasionally visit the colony, flying low over it and often alighting to preen or roost on the sandy foreshore. Visits increase around midday and then follows a period of preening and roosting. During the afternoon the terns gradually disappear, and they come in to roost only as darkness falls. It has been suggested that at this stage they spend most of the day fishing because of the scarcity of food in the surface layers of inshore waters, but this diurnal pattern is very similar to that which I have seen in West Africa during the two months prior to their departure north. Before mid-April there are terns at the colony throughout the day, but with a small peak in numbers around midday and early afternoon and a big influx at dusk. The Sandwich Tern's habit of nesting with more belligerent species, such as Black-headed Gulls Larus ridibundus, is well docu• mented. Many years' study have shown that in northern Scotland it prefers to nest in early May, when its breeding success is likely to be enhanced. Perhaps for this reason, it tends to synchronise more readily with an early nester such as the Black-headed Gull, which is on the colony from early March and lays in mid-April, than with the smaller terns which do not begin laying until mid- May. The influence of nesting Black-headed Gulls on Sandwich Terns in search of breeding sites has been discussed by a number of writers, such as Salomonsen (1947), Van den Assem (1954) and Isenmann (1972). At Forvie, from mid-April until laying time, the Sandwich Tern flock has in recent years moved at dusk into the Black-headed Gull colony until it is protected on all sides. This may be primarily for protection from ground predators, but it is possible also that the terns use the situation to gauge the suitability of the area for breed• ing. Although there are disadvantages in this association, Lind (1963) considered that the advantages were greater. The terns certainly benefit from the greater vigilance and aggressiveness of the gulls, but Rooth (1958) thought that the latter 'may be harmful to the tern populations on account of space competition, food parasitism, predation of eggs and predation of young birds'. In the Camargue, France, no predation of eggs and young was recorded in a mixed colony of Sandwich Terns and Black-headed Gulls (Isenmann 1972). Although gull predation of eggs and young occurs at Sands of Forvie, it does not seriously affect breeding success (though see pages 150-151). COURTSHIP Colour-ringing at Forvie has shown that among the early arrivals 144 Sandwich Tern studies there are paired birds, some of which have been identified in pre• vious years. One pair nested in four successive seasons and, once paired, usually in the third or fourth year, the Sandwich Tern may well pair for life. Many other early arrivals are unattached or have poorly developed pair bonds, while later arrivals are almost always paired, though in mid-June there is an influx of unpaired two- and three-year-old birds (see pages 152-153). At first courtship is latent, especially in the earliest arrivals, but it requires only the appearance of a newcomer or a blink of sunshine to set off ground or aerial courtship. At this stage, however, particularly in the overcast weather of northern Scotland, it often quickly wanes. Prolonged sunshine encourages a continuation of courtship, especially aerial courtship. The typical 'high flight' (see below) has been described in considerable detail by Van den Assem (1954) and Cullen (1960a). It may be started by two or three (occasionally four or five) birds which have just flown in from the sea and arrived over the colony, or by members of the flock on the shore near the colony, or by a male in advertisement flight over the flock. The collective 'upflight' (Lind 1963), or rising up of the whole flock, is also often seen at this stage, precipitated by, for example, a movement of the tide, a corvid flying near, or the approach of a human being. Lind suggested that such com• munal flights may well influence the frequency of sexual behaviour, and certainly many high flights originate from them. The earliest high flight I saw in Ghana was triggered by an upflight when a Black Kite Milvus migrans flew over the flock roosting at a lagoon. An unmated male Sandwich Tern attracts the attention of the females by flying around over the flock, carrying a fish and calling a characteristic 'koreet'. Occasionally no fish is carried and the call then sounds more like 'kireet' or 'kirrik'. This is the male's advertisement flight. Alternatively he may alight in or near the flock and approach several birds, calling and raising his head and bill. Usually the wings are held away from the body and the crest raised in excitement. When he has the attention of a female, which may be when she also adopts the courtship display (plate 17), he takes off in an aerial 'bent posture', with head and bill pointed downwards and back slightly arched, and she follows him into the high flight. Other birds may join them. Circling, they climb rapidly until sometimes they are almost invisible even with powerful binoculars. Before they reach this height, the original male may be left with his female, but occasionally in the initial stages three, four or even five birds may stay together throughout and break up only after the fast downward 'glide' which often follows several false starts. The high flight may persist for some time, individual birds participating for up to ten minutes or longer before going Sandwich Tern studies 145 into a glide. In the glide the birds may fly very close, and on several occasions I have seen them touch at the 'pass' when one overtakes another, which happens a number of times during the descent. The overtaken bird goes into the bent posture described above, while the other, if paired to it, appears normal (Cullen 1960a); but where a number of birds are participating I have seen both go into the bent posture at the pass. The overtaken bird, especially if carrying a fish, may also raise its wings over its back, sometimes beating them slightly; this has been termed 'V-flying', which describes the action well. The calls during the high flight are characteristic. During the upward part both birds give 'koreet', 'kek', and 'arrie' or 'erre' calls (Van den Assem 1954), and R. Chestney (verbally) thinks that the sexes can thus be differentiated, the female calling noticeably higher than the male. During the glide a quickly repeated 'kekekekekekekek' is uttered by both birds, especially the bent one at the pass. Following the glide, and depending on the development of the pair bond, the participants may break off to fish, preen, or go into ground courtship or often into another display flight described very well by Van den Assem (1954): 'The flight occurs in a particu• lar way: the wings are thrust emphatically through the beat, by which the body without actually advancing more quickly gets a push upwards so that the flight looks like a dance'.